Solenoid force motors, as used in fluid pressure control applications, are designed to supply pressure in direct proportion to the average current supplied to the solenoid coil. This means that a control system operating the force motor must schedule the supplied current as a function of the desired fluid pressure.
From a control system standpoint, the simplest approach is to maintain strict tolerances on the force motor impedance, and control the current in open-loop fashion. However, strict tolerancing of the force motor impedance is impractical in applications where the solenoid portion of the motor is subject to wide temperature excursions. The present disclosure is an example of such an application the force motor is employed for vehicle transmission pressure control, and is completely submersed in transmission fluid, which varies over the approximate range of -40.degree. C. to +150.degree. C. in normal operation. In such an application, impedance variations of up to 260% may be observed. In addition, source voltage variations may present a significant concern.
In applications such as the one described above, the conventional solution is to employ closed-loop control of the current--that is, measuring the average current actually supplied to the force motor, comparing it to the commanded current, and adjusting the current supply by an amount determined in relation to the difference, or error. In theory, the measured current will reflect the impedance of the force motor, and the current supply adjustment will compensate for any changes in the impedance. However, the measured current must reflect the average current, which generally requires substantial filtering of the measured (instantaneous) current, especially where the current is controlled by pulse-width-modulation (PWM) of the applied voltage. Unfortunately, the filtering, if sufficient to provide a measure of the average current, creates a pole which tends to interact with the pole of the force motor. This produces an underdamped transient response characteristic, as observed by overshooting of the commanded current.